Macromolecules 2002, 35, 1611-1621
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Control of the Radical Polymerization of tert-Butyl Methacrylate in Water by a Novel Combination of Sodium Nitrite and Iron(II) Sulfate C. Detrembleur,† Ph. Teyssie´ , and R. Je´ roˆ me* Center for Education and Research on Macromolecules (CERM), University of Lie` ge, Sart-Tilman, B6, 4000 Lie` ge, Belgium Received July 30, 2001
ABSTRACT: Addition of iron(II) sulfate to sodium nitrite is a new way to control the radical polymerization of tert-butyl methacrylate (tBMA) in water at 80 °C. Nitric oxide is released in the reaction medium as result of the reduction of sodium nitrite by FeSO4. This reaction is of utmost importance because three repeated additions of an alkyl radical to nitric oxide leads to the formation of nitroxide radicals. The basic mechanism for the control of the radical polymerization is thus a nitroxide-mediated process.
Introduction For a long time, anionic polymerization was the only polyaddition reaction known for livingness and, thus, for its capacity to design synthetic polymers and to tune their molecular characteristics in close relation to the envisioned application.1 The implementation of anionic polymerization however requires drastic experimental conditions (extreme dryness, high purity of the monomer and solvent, usually low temperature, etc.), which limits large-scale production. Most polymers are commercially produced by free-radical polymerization, because of much less demanding conditions and the wide range of monomers which can be (co)polymerized. The neutrality of the radical species is however responsible for irreversible transfer and termination reactions, at the origin of uncontrolled polymerization. Recently, the controlled radical polymerization of vinylic and (meth)acrylic monomers has been reported, which relies upon a fast exchange between growing radicals (active species) and inactive (or dormant) species. If this equilibrium is shifted enough toward the dormant species, the stationary concentration of the active species is low, which strongly limits the extent of the chain breaking reactions. At the time being, three main successful approaches have been reported. The first one, known as the nitroxyl-mediated polymerization (NMP), is based on the reversible coupling of the propagating radical with a stable free radical. This system is well suited to the (co)polymerization of styrene and derivatives.2 It was recently extended to the polymerization of acrylates3 and 1,3-dienes.4 Nevertheless, the control of the polymerization of methacrylates is still a challenge. The atom transfer radical polymerization (ATRP) is the second strategy which promotes the reversible termination by the transfer of a ligand from a metal complex to the propagating radical, and vice versa. This method is effective in the polymerization of mainly (meth)acrylates and styrene derivatives.5 The extension of this control to polymerization conducted in aqueous media6 and particularly to emulsion polymerization must be noted for both the NMP7 and the ATRP8 methods. Finally, Rizzardo et al.9 have reported on a †
Present address: Bayer AG, Leverkusen, Germany. * To whom correspondence should be addressed.
new mechanism based on the reversible additionfragmentation chain transfer (RAFT process). This controlled radical polymerization has the advantage to be compatible with a wide range of monomers, including monomers that contain functional groups, e.g., acid, acid salt, hydroxyl, and tertiary amine, and with aqueous media. This paper aims at reporting on the preliminary results about the controlled radical polymerization of tert-butyl methacrylate (tBMA) in water using sodium nitrite as a mediator. The nitrite chemistry will be discussed in order to shed light on the polymerization mechanism and to account for the main experimental observations. Experimental Section Materials. tBMA from BASF was distilled just before use in order to remove the stabilizer. Bidistilled water was degassed prior to use by bubbling nitrogen for 30 min. K2S2O8 (Aldrich), FeSO4‚7H2O (Aldrich), AIBN (Aldrich), and NaNO2 (Backer) were used as received. Liquids were transferred under nitrogen by syringes or through stainless steel capillaries. Polymerization. a. The NaNO2/FeSO4/K2S2O8 System. In a typical experiment, 0.0207 g of NaNO2 (3 × 10-4 mol) and 0.0834 g of FeSO4‚7H2O (3 × 10-4 mol) were added to the reaction flask, which was evacuated by three nitrogenvacuum cycles. 15 mL of degassed water and 8 mL of tBMA (3.14 × 10-2 mol) were then added, and the flask was thermostated at 80 °C. Finally, 5 mL of a solution of 0.811 g of K2S2O8 (3 × 10-3 mol) in 50 mL of water was added. Samples were regularly withdrawn from the polymerization medium, and the monomer conversion was calculated by 1H NMR analysis of the organic phase. Polymerization was stopped after 24 h, thus when the medium viscosity was too high for additional samples to be picked out. b. The NaNO2/FeSO4/AIBN System. In a typical experiment, 0.0414 g of NaNO2 (6 × 10-4 mol), 0.0834 g of FeSO4‚ 7H2O (3 × 10-4 mol), and 0.0492 g of AIBN (3 × 10-4 mol) were added to the reaction flask, which was evacuated by three nitrogen-vacuum cycles. 20 mL of degassed water and 8 mL of tBMA (5 × 10-2 mol) were then added, and the flask was thermostated at 80 °C. Samples were regularly withdrawn and analyzed as described above. c. Synthesis and Purification of the PtBMA Oligomers Synthesized by the NaNO2/FeSO4/K2S2O8 System and Analyzed by ESR. 0.207 g of NaNO2 (3 × 10-3 mol) and 0.834 g of FeSO4‚7H2O (3 × 10-3 mol) were added to the reaction flask, which was evacuated by three nitrogen-vacuum cycles. 75 mL of degassed water and 40 mL of tBMA (0,25 mol) were
10.1021/ma011368z CCC: $22.00 © 2002 American Chemical Society Published on Web 01/31/2002
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Figure 1. Dependence of Mn,SEC on the tBMA conversion, in the case of initiation by AIBN in water at 80 °C in the presence or not of NaNO2 and FeSO4 [2 equiv of NaNO2, 1 equiv of FeSO4, 1 equiv of AIBN, 168 equiv of tBMA, water/tBMA ) 2.5]. then added, and the flask was thermostated at 80 °C. Finally, 25 mL of a solution of 0.811 g of K2S2O8 (3 × 10-3 mol) in 50 mL of water was added. One hour later, the monomer conversion was calculated by 1H NMR analysis of the organic phase (monomer conversion: 6.5), which indicated a very poor control of the reaction. According to Bortel et al., sodium nitrite is reversibly decomposed in aqueous media in a way which is strongly dependent on temperature and pH (eq 1).
2NaNO2 + H2O h NO + NO2 + 2NaOH
Figure 9. Volumetric measurement of NO formed by reaction of NaNO2 with FeSO4 in water at 80 °C [10-3 mol of NaNO2, 10-3 mol of FeSO4, 33.5 mL of water, 80 °C].
toward the dormant species with the polymerization progress. So, keeping constant the amounts of monomer, potassium persulfate, and FeSO4, the molecular weight of the polymer is dictated by the amount of nitrite, which should not be too low for decreasing the extent of the irreversible termination reactions. Preliminary Approach of the Mechanism. In the scientific literature, NaNO2 is a well-known inhibitor for the polymerization of water-soluble monomers at room temperature. Bortel et al.13 speculated that the free radical polymerization of the sodium salt of acrylic acid in aqueous solution at 90 °C could be at least partly controlled by nitroxide radicals released from NaNO2. This polymerization initiated by ammonium persulfate
(1)
Nitric oxide has one odd electron in an antibonding π orbital, whereas nitrogen dioxide contains 17 electrons with one electron unpaired in a nitrogen or an oxygen orbital. The propagating chains can thus be trapped by these NO and NO2 paramagnetic species, with formation of nitro compounds and nitroso compounds, respectively. The nitro compound can be further transformed into a nitroso compound by reaction with an additional radical. According to these authors, the APS radicals and the growing polymer radicals would be capped by nitroxide radicals in a dynamic equilibrium between active and dormant species, although no structure was proposed for the nitroxide radicals. Formation of nitroso compounds by reaction of nitric oxide (NO) with free alkyl radicals has been reported independently by several authors14,15 and formation of complexes of NO with transition-metal ions as well.16 Moreover, the fast reduction of sodium nitrite by an acidic solution of FeSO4 is known to release NO (eq 2).17
2NaNO2 + 2FeSO4 + 3H2SO4 f Fe2(SO4)3 + 2NaHSO4 + 2H2O + 2NO (2) This reaction also occurs at neutral or slightly basic pH, although much more slowly. Because the tBMA polymerization was carried out under the same conditions
Scheme 1
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Detrembleur et al. Scheme 2
as eq 2 except for the addition of sulfuric acid, the formation of NO may be postulated. To confirm this hypothesis, degassed solutions of FeSO4 and NaNO2 (0.03 M) were mixed and heated at 80 °C for a few minutes. An important release of NO2 was observed when the flask was opened, as result of the direct oxidation of NO by air. Furthermore, the reaction medium became brown, in line with oxidation of iron(II) by sodium nitrite. Finally, the rate of the NO formation was measured by volumetry under the same conditions as used for the tBMA polymerization (same concentrations of NaNO2 and FeSO4). Figure 9 shows that most of NO is formed within a few minutes and that the reaction is quantitative after 15 min. Under the same conditions, but without any iron sulfate, only 4.5% of the theoretical amount of NO is formed after 30 min, and no more gas is evolved later on. From this discussion, nitric oxide appears to be the key actor in the control of the tBMA radical polymerization. The main role of iron(II) sulfate would be to reduce rapidly sodium nitrite into nitric oxide which controls the polymerization. To understand better the polymerization mechanism, the chemistry of nitric oxide and related derivatives in radical reactions is discussed hereafter. Reaction of Nitric Oxide with Free Radicals. As aforementioned, nitroso compounds are formed as result of reaction of nitric oxide with alkyl radicals.14,18-22 These nitroso compounds are extensively used as spintrapping agents in the study of short-lived radicals by ESR. Indeed, these radicals add to nitroso compounds and form long-lived nitroxide radicals well-suited to detection by ESR.15,22-24 Sato and Otsu reported on the spin-trapping of a series of short-lived organic radicals by the 2-methyl-2-nitrosopropane with formation of the parent nitroxide radicals.23 In 1953, Waters et al.14 studied the reaction of nitric oxide with 2-cyano-2-propyl free radicals, •CMe2-CN, released by decomposition of AIBN in boiling benzene (Scheme 1). A deep green color was first observed, which was indicative of a tertiary nitroso compound. When the reaction was complete, the product turned out to be the colorless alkoxyamine formed by the addition of three alkyl residues to the nitric oxide molecule (Scheme 1). The authors also showed that the 2-cyano-2-propyl radicals added rapidly to nitrosobenzene to form the corresponding alkoxyamine. Recently, Vertommen et al.18 and Nesvadba et al.19 patented the synthesis of tertiary alkoxyamines by the reaction of nitric oxide with alkyl radicals and their use as initiators for the controlled radical polymerization of vinyl monomers, although the control of the polymerization remained questionable. In a more recent paper, Nesvadba successfully reported on the efficiency of new alkoxyamines, prepared by addition of free radicals to nitroso compounds, to control the polymerization of styrene.20 Reaction of Nitric Oxide with Functionalized Alkenes.25,26 It is known that pure NO does not add to alkenes substituted by an acceptor or a donor group, no β-nitrosoalkyl radical being formed. However, whenever NO2 is present, it is added to the less sterically
Macromolecules, Vol. 35, No. 5, 2002 Scheme 3
Scheme 4
hindered carbon of the double bond with formation of β-nitroalkyl radicals. The subsequent addition of NO to these radicals leads to R-nitroso-β-nitro compounds according to Scheme 2. These reactions also occur with alkyl methacrylates, and Shechter et al.27 were the first to report on the synthesis of 2-methyl-3-nitro-2-nitrosomethylpropionate (O2N-CH2-C(CH3)(CO2Me)(NO); NMMA) by reaction of methyl methacrylate (MMA) with a sodium nitrite/ sulfuric acid mixture (this mixture being a precursor of a NO and NO2 mixture). Furthermore, many authors detected a three-line ESR spectrum when MMA was reacted with NO in the presence of NO2 at room temperature,28 this spectrum being the signature of nitroxide radicals. Scheme 3 shows that these radicals 3 result from the addition of 2-methyl-3-nitromethylpropionate radicals 1 to methyl 2-methyl-3-nitro-2-nitrosopropionate 2. As shown in Scheme 4, the R-nitroso-β-nitrostyrenes 5 were formed by bubbling a NO/NO2 mixture (1/1 molar ratio) in a styrene solution.22 Nitroxide radicals 6 were detected, which resulted from the reaction of the β-nitro styrene radicals 4 with the R-nitroso-β-nitro styrene 5. Mechanism of the Controlled Radical Polymerization of tBMA Initiated by the NaNO2/FeSO4/ Initiator System (Scheme 5). At the early stage of the polymerization, NO is formed by the rapid reduction of sodium nitrite by FeSO4. The initiating radicals (2cyano-2-propyl radicals, R•) or the propagating chains (P•) are trapped by nitric oxide and form nitroso
Macromolecules, Vol. 35, No. 5, 2002
Polymerization of tBMA in Water 1617 Scheme 5
compounds (R-NO and P-NO, respectively) as supported by the observation of a deep green-blue color. Later on, this color disappears, consistent with the addition of new radicals to the nitroso compounds and formation of nitroxide radicals. These nitroxide radicals reversibly trap the propagating radicals, thus leading to a dynamic equilibrium between dormant and active species, which is thought to be the key step for the control observed. Three initiating/propagating radicals per NO molecule are thus needed to form the alkoxyamine which would be responsible for the control. It is difficult to estimate the amount of NaNO2 and initiator required to form quantitatively the alkoxyamine because the initiator efficiency strongly depends on solvent, temperature, and dilution and because NO has a limited solubility in the liquid phase and can be lost partly when samples are picked out from the polymerization flask. Therefore, formation of the alkoxyamine may be not complete.
Moreover, formation of NO2 by reaction of the “in situ” formed NO2 with residual oxygen may not be precluded.25 NO2 can react with tBMA with formation of the parent β-nitroalkyl radical, which either initiates the polymerization or is trapped by NO and forms a nitroso compound and ultimately nitroxide radical (see Schemes 2 and 3). To prove that nitrogen oxides add to tBMA, the gas formed by reacting equimolar solutions of sodium nitrite and iron(II) sulfate was monitored in the presence or not of tBMA (Figure 10). As previously reported, NO is quantitatively formed after 15 min at 80 °C. However, in the presence of tBMA, NO is rapidly formed at least until the release of 70% of the theoretical amount after 3.5 min. Then, the volume of gas decreases and tends to a plateau of 30-35% of the theoretical amount. So 65% of the gas (NO and NO2) is consumed by the monomer, as qualitatively attested by the green-blue color characteristic of nitroso compounds. After 40 min
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Macromolecules, Vol. 35, No. 5, 2002 Scheme 6
Figure 10. Volumetric measurement of NO formed by reaction of NaNO2 and FeSO4 in water at 80 °C, in the presence or not of tBMA [10-3 mol of NaNO2, 10-3 mol of FeSO4, 33.5 mL of water, 6.3 × 10-2 mol of tBMA, 80 °C].
of reaction, the organic phase was isolated and washed several times with water, and the residual monomer was eliminated under vacuum at room temperature. About 0.16 g of a yellow-brown viscous solid was collected (from 8.94 g of tBMA). To elucidate the structure of this residue, the IR spectrum was recorded and compared to the IR spectra of tBMA and poly(tBMA) (Figure 11). The IR spectrum of the residue does not fit tBMA but rather conventional poly(tBMA). The additional peak observed at 1561 cm-1 might be the signature of the NO2 group. However, as will be reported further, nitroso compounds have a strong tendency to form dimers, which makes the assignment of the
absorption peak of the NO group quite uncertain, because of its dependence on the unimer vs dimer structure. The 1H NMR spectrum of the residue (Figure 12) confirms the similarity of structure with poly(tBMA), as attested by the characteristic peaks associated with poly(tBMA) in the 2.0-0.8 ppm range.29 Additional peaks are observed in the 4.0-6.8 ppm range, which may be assigned to the chain ends. The singlets observed at 5.42 and 5.95 ppm are typical of the vinylidene groups (present at the chain end of some dead PtBMA chains), and the two multiplets at 4.2 and 4.6 ppm may be assigned to the CH2 of the poly(tBMA) backbone bonded to the NO2 group. All these analytical data (IR, 1H NMR, volumetric measurements) confirm the addition of nitrogen oxide to tBMA and formation of poly(tBMA) oligomers endcapped by NO2 and more likely by NO, in agreement with the scientific literature.25,26 Scheme 6 shows the structure of the residue which is expected to be formed by reaction of sodium nitrite and iron(II) sulfate with tBMA. In addition to the nitroso compound, the parent nitroxide radical may be formed by the addition of a nitroalkyl radical as shown in Scheme 7. Finally, to assess further the mechanism, the electron spin resonance (ESR) spectrum was recorded at 80 °C for oligomers of PtBMA (Mn ) 1400, Mw/Mn ) 1.03) synthesized by the NaNO2/FeSO4/K2S2O8 system and purified before analysis (Figure 13). A triplet with AN
Figure 11. IR spectra of (A) the residue isolated after reaction of sodium nitrite and FeSO4 with tBMA, (B) tBMA, and (C) poly(tBMA).
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Figure 12. 400 MHz 1H NMR of the reaction product of sodium nitrite with iron(II) sulfate in the presence of tBMA: (A) whole spectrum, (B) 3.0-7.5 ppm extended spectrum (*: vinylidene units in dead PtBMA chains). Scheme 7
) 13.7 G and a g factor ) 2.0082 are observed. This basic triplet, characteristic of nitroxide radicals,30 confirms the formation of these species in the polymerization medium. The three-line ESR spectrum is a consequence of the hyperfine coupling to nitrogen and confirms that the nitrogen atom of the nitroxide radical is bonded to two tertiary carbons. Indeed, a more complex ESR spectrum should be observed in the case of bonding of the nitrogen atom to a secondary and/or primary carbon, as result of coupling of the radical to the hydrogen atoms bonded to these carbons. This spectrum is thus in good agreement with the structure for the nitroxide radical expected to be formed in the polymerization medium (see Scheme 5). Consistent with the key role of NO and NO2, the molecular weight of poly(tBMA) must decrease when the
amount of NaNO2 is increased with respect to the other reagents, in agreement with the experimental data in Figure 7. When the NaNO2/K2S2O8 molar ratio is 0.5/1, only one NO molecule is generated compared to four sulfate radicals. The exceeding radicals, which cannot form the alkoxyamine, are killed rapidly at the expense of the initiator efficiency. When the NaNO2/K2S2O8 molar ratio is increased from 1/1 to 2/1, the molecular weight decreases, although an excess of NO is formed with respect to the radicals in both the cases (on the assumption that one NO reacts with three radicals). This observation might be explained by the noncomplete dissolution of NO in the liquid phase. Moreover, more NO and NO2 are formed when the relative amount of NaNO2 is increased, such that more chains are initiated by NO2 and their molecular weight decreases.
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in the frame of the “Poˆles d’Attraction Interuniversitaires: PAI 4/11”. They are also grateful to Dr. A. Mouithys-Mickalad (CORD, University of Lie`ge) for assistance in the ESR measurements and to Mrs F. Luthers for her skillful assistance. References and Notes
Figure 13. ESR spectrum, in toluene at 80 °C, for poly(tBMA) formed in water by the NaNO2/FeSO4/K2S2O8 system [5 mg of poly(tBMA) in 0.5 mL of toluene].
Conclusions Sodium nitrite is quite an inexpensive compound which can impart control to the radical polymerization of tBMA initiated by AIBN or potassium persulfate in water at 80 °C. A low amount of FeSO4 is however required to observe a reasonable control, particularly when AIBN is the initiator. The molecular weight of poly(tBMA) is decreased when the amount of NaNO2 is increased, the amount of the other constituents being kept constant. Consistent with the scientific literature and the experiments carried out in this work, a four-step mechanism can be proposed (Scheme 5): (i) formation of nitric oxide by reduction of NaNO2 by FeSO4; (ii) reaction of propagating radicals with NO and formation of nitroso compounds; (iii) further reaction of propagating radicals with the nitroso compounds and formation of the parent nitroxide radicals; (iv) reversible trapping of the growing chains by the nitroxide radicals, which leads to a dynamic equilibrium between active and dormant species, at the origin of the polymerization control. Additionally, tBMA can react with NO2 formed in parallel to NO. The β-nitroalkyl radicals accordingly formed can initiate the polymerization and/or form nitroso compounds. Although the control is imparted to the polymerization of tBMA by the nitrite system in water, some side reactions occur as shown by deviation of ln([M]0/[M]) vs time from linearity, and the increase in polydispersity when the monomer conversion is increased. Several causes can contribute to the gradual loss of control, i.e., the heterogeneity of the system, the number of steps leading to the dormant species with opportunity of side reactions at each step, formation of several nitroso and nitroxide compounds of a different reactivity, and the not yet optimized amounts of NaNO2 and initiator. Nevertheless, this system is quite a simple method to control the radical polymerization of alkyl methacrylates in aqueous medium. It is important to note that the nitroxide-mediated polymerization (NMP) is usually known for a poor control of the polymerization of alkyl methacrylates. Therefore, the nitrite system raises new prospects in this field. Acknowledgment. The authors are much indebted to the “Services Fe´de´raux des Affaires Scientifiques, Techniques et Culturelles” for general support to CERM
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